It is known that when two ends of one or two hard articles, for example, steel articles or other metal articles, are bonded or welded together, the peripheral surface of the bonded portion of the resultant hard article is covered with a thermoplastic covering sheet so as to protect the bonded portion from corrosion and/or rusting. Especially, in the fields of coated steel pipes, tubes, bars and cables, it is extremely important to completely protect the bonded portion thereof from corrosion and/or rusting by firmly covering the bonded portion with a fluid-impermeable thermoplastic covering sheet.
Also, it is known that, for the purpose of protecting the bonded portion of the hard article, the peripheral surface of the bonded portion should be wrapped with a thermoplastic covering sheet which may be in the form of a tape or tube, and which has an adhesive layer formed on a surface of the sheet, in such a manner that the adhesive layer comes into contact with the peripheral surface of the bonded portion and that the two end portions of the covering sheet on the hard article overlap each other; the overlapping end portions should be temporarily bonded to each other; and the covering sheet should be heated at a high temperature sufficient to cause the covering sheet to shrink, whereby the covering sheet comes into close contact with the peripheral surface of the bonded portion of the hard article and is firmly bonded thereto with the adhesive layer of the covering sheet.
The heating procedure of the covering sheet may be carried out either externally or internally. In the external heating procedure, the covering sheet applied onto the hard article is heated from the outside thereof by using a heater or a gas burner. In the internal heating procedure, an electroconductive heat-generating element is incorporated in the body of the covering sheet, and, after the covering sheet is wrapped around the bonded portion of tbe hard article, an electric current is applied to the heat-generating element.
The above-mentioned conventional covering process is advantageous in that the covering procedure can be completed within a short period of time, and the protecting effect of the covering sheet on the hard article is substantially satisfactory.
However, the above-mentioned conventional covering process is disadvantageous in that undesirable air bubbles are formed between the covering sheet and the surface of the hard article. In the usual application of the covering sheet onto the hard article, the covering sheet is wrapped around the peripheral surface portion of the hard article to be protected, in such a manner that the adhesive layer of the covering sheet faces the peripheral surface portion of the hard article; then, the covering sheet is heated to an extent that the covering sheet shrinks, so that the adhesive layer becomes mobile and comes into close contact with the peripheral surface portion of the hard articles; then the adhesive layer is solidified while the covering sheet presses the hard article under a pressure generated due to the shrinkage thereof. When the covering sheet is wrapped around the hard article, a certain amount of air is retained between the covering sheet and the peripheral surface of the hard article. Usually, the retained air is not completely removed during the heat-fastening procedure of the covering sheet and, thus, air bubbles are formed in the adhesive layer or between the adhesive layer and the peripheral surface of the hard article. The air bubbles cause the bonding strength of the fastened covering sheet to the peripheral surface of the hard article to be unsatisfactory and, therefore, the protecting effect of the covering sheet for the hard article is poor. Accordingly, it is important to remove the air bubbles from between the covering sheet and the bonded portion of the hard article. This removal of the air bubbles is carried out manually by moving a hand or a pressing pad from a central portion of the peripheral surface of the fastened covering sheet toward a side edge portion thereof while pressing the peripheral surface of the covering sheet with the palm of the hand or pressing pad.
The above-mentioned manual removal of the air bubbles is disadvantageous in the following points.
1. A long time period is necessary to complete the removal of the air bubbles.
2. The worker should be skilled in the removing operation.
3. It is difficult to completely remove the air bubbles, especially in the case where the hard article is of a large size, for example, in the case of a large steel pipe for an oil pipe line.
Accordingly, it is desirable to provide a new process for fastening the covering sheet on the peripheral surface portion of a hard article while removing the air bubbles formed therebetween.